When changes are made in substances by the addition of other substances, they make what is called a chemical union; and in this case the properties of the compound cannot with any certainty be deduced from those of the component parts, but must be ascertained by fresh experiments. Thus, from the specific gravities, or the degrees of fusibility, of two metals, those of the compound cannot be predicted. Neither water nor acid of vitriol will separately dissolve iron, so as to produce inflammable air, but both together will do it. However, the properties of similar compounds are similar to one another. Thus, all metals dissolved in acids are precipitated by mild alkalis. This chemical union of two substances we ascribe to a certain elective attraction, or affinity that subsists between them, in consequence of which they unite with one another whenever a proper opportunity offers, in preference to those substances to which they were before united. Thus the vitriolic acid, having a stronger affinity with the vegetable alkali which is the basis of nitre, will unite with that alkali, and with it form another compound, called vitriolated tartar, while the acid of nitre, being detached from its base, is collected separately.
When two substances compose one liquid, and a third, which has a stronger affinity with either of the two parts than they have with each other, is added to them, it will unite with that part, and take its place in the solution, while the other will in many cases be precipitated, and may be collected. Thus the earth of alum is precipitated from a solution of alum by salt of tartar. This is the case of simple affinity.
When both the substances are compounds, the component parts of which have a weaker affinity with each other than they have with those of the other compound, two new combinations are formed, and this is called a case of double affinity. Thus when phlogisticated alkali is poured into a solution of green vitriol, the acid of the vitriol unites with the alkali, while the phlogiston joining the calx of iron makes Prussian blue.
All nature lying open to our investigation, we must consider the different parts in some order. But it is not very material which we adopt, because, begin where we will, the properties of the substances we first treat of will be connected with those which must be particularly considered afterwards, the changes in one substance being occasioned by its union with another. It will be impossible, for example, to explain the properties of metals without considering the acids, because by their union with acids very important changes are made in their properties.
There have been three principal methods of arranging natural substances. One is according to the three kingdoms, as they are called, into which they have been distributed, viz. the mineral, the vegetable, and the animal. Another is according to the elements which enter into their composition, and a third according to the form in which they are usually found, viz. aerial, fluid, or solid. Upon the whole this last appears to me to be the most convenient, especially as it is easy to intermix general observations concerning the other divisions when they are particularly wanted. As there will be frequent occasion to speak of the component and elementary parts of all substances, I shall here observe, that, according to the latest observations, the following appear to be the elements which compose all natural substances, viz. dephlogisticated air, or the acidifying principle; phlogiston, or the alkaline principle; the different earths, and the principles of heat, light, and electricity. Besides these, there are the following principles which have not been proved to be substances, viz. attraction, repulsion, and magnetism. By the help of these principles we are able, according to the present state of natural knowledge, to explain all the appearances that have yet occurred to us.
LECTURE II.
Of the Properties of all Matter.
Before I consider the properties of particular substances, it will be proper to mention those which are common to them all. But I shall first observe, that the term substance has no proper idea annexed to it, but is merely a convenience in speech; since we cannot speak of the properties of substances, such as hard, round, coloured, &c. &c. (which circumstances alone affect our senses, and thereby give proper ideas) without saying that they inhere in, or belong to, some thing, substance, or substratum. The terms being and person are also in the same predicament.
One property of all substances is extension, since they all occupy some portion of space.